AHT20 Humidity + Temperature Sensor

rkorell · Apr 16, 2025, 9:58 AM

the python script you are looking for is bme280.py

#!/usr/bin/python
#--------------------------------------
#    ___  ___  _ ____
#   / _ \/ _ \(_) __/__  __ __
#  / , _/ ___/ /\ \/ _ \/ // /
# /_/|_/_/  /_/___/ .__/\_, /
#                /_/   /___/
#
#           bme280.py
#  Read data from a digital pressure sensor.
#
#  Official datasheet available from :
#  https://www.bosch-sensortec.com/bst/products/all_products/bme280
#
# Author : Matt Hawkins
# Date   : 21/01/2018
#
# https://www.raspberrypi-spy.co.uk/
#
#--------------------------------------
import smbus
import sys
import time
from ctypes import c_short
from ctypes import c_byte
from ctypes import c_ubyte

DEVICE = 0x76 # Default device I2C address

try: #override device address like '0x77'
    DEVICE = int(sys.argv[1], 16)
except:
    pass

bus = smbus.SMBus(1) # Rev 2 Pi, Pi 2 & Pi 3 uses bus 1
                     # Rev 1 Pi uses bus 0

def getShort(data, index):
  # return two bytes from data as a signed 16-bit value
  return c_short((data[index+1] << 8) + data[index]).value

def getUShort(data, index):
  # return two bytes from data as an unsigned 16-bit value
  return (data[index+1] << 8) + data[index]

def getChar(data,index):
  # return one byte from data as a signed char
  result = data[index]
  if result > 127:
    result -= 256
  return result

def getUChar(data,index):
  # return one byte from data as an unsigned char
  result =  data[index] & 0xFF
  return result

def readBME280ID(addr=DEVICE):
  # Chip ID Register Address
  REG_ID     = 0xD0
  (chip_id, chip_version) = bus.read_i2c_block_data(addr, REG_ID, 2)
  return (chip_id, chip_version)

def readBME280All(addr=DEVICE):
  # Register Addresses
  REG_DATA = 0xF7
  REG_CONTROL = 0xF4
  REG_CONFIG  = 0xF5

  REG_CONTROL_HUM = 0xF2
  REG_HUM_MSB = 0xFD
  REG_HUM_LSB = 0xFE

  # Oversample setting - page 27
  OVERSAMPLE_TEMP = 2
  OVERSAMPLE_PRES = 2
  MODE = 1

  # Oversample setting for humidity register - page 26
  OVERSAMPLE_HUM = 2
  bus.write_byte_data(addr, REG_CONTROL_HUM, OVERSAMPLE_HUM)

  control = OVERSAMPLE_TEMP<<5 | OVERSAMPLE_PRES<<2 | MODE
  bus.write_byte_data(addr, REG_CONTROL, control)

  # Read blocks of calibration data from EEPROM
  # See Page 22 data sheet
  cal1 = bus.read_i2c_block_data(addr, 0x88, 24)
  cal2 = bus.read_i2c_block_data(addr, 0xA1, 1)
  cal3 = bus.read_i2c_block_data(addr, 0xE1, 7)

  # Convert byte data to word values
  dig_T1 = getUShort(cal1, 0)
  dig_T2 = getShort(cal1, 2)
  dig_T3 = getShort(cal1, 4)

  dig_P1 = getUShort(cal1, 6)
  dig_P2 = getShort(cal1, 8)
  dig_P3 = getShort(cal1, 10)
  dig_P4 = getShort(cal1, 12)
  dig_P5 = getShort(cal1, 14)
  dig_P6 = getShort(cal1, 16)
  dig_P7 = getShort(cal1, 18)
  dig_P8 = getShort(cal1, 20)
  dig_P9 = getShort(cal1, 22)

  dig_H1 = getUChar(cal2, 0)
  dig_H2 = getShort(cal3, 0)
  dig_H3 = getUChar(cal3, 2)

  dig_H4 = getChar(cal3, 3)
  dig_H4 = (dig_H4 << 24) >> 20
  dig_H4 = dig_H4 | (getChar(cal3, 4) & 0x0F)

  dig_H5 = getChar(cal3, 5)
  dig_H5 = (dig_H5 << 24) >> 20
  dig_H5 = dig_H5 | (getUChar(cal3, 4) >> 4 & 0x0F)

  dig_H6 = getChar(cal3, 6)

  # Wait in ms (Datasheet Appendix B: Measurement time and current calculation)
  wait_time = 1.25 + (2.3 * OVERSAMPLE_TEMP) + ((2.3 * OVERSAMPLE_PRES) + 0.575) + ((2.3 * OVERSAMPLE_HUM)+0.575)
  time.sleep(wait_time/1000)  # Wait the required time  

  # Read temperature/pressure/humidity
  data = bus.read_i2c_block_data(addr, REG_DATA, 8)
  pres_raw = (data[0] << 12) | (data[1] << 4) | (data[2] >> 4)
  temp_raw = (data[3] << 12) | (data[4] << 4) | (data[5] >> 4)
  hum_raw = (data[6] << 8) | data[7]

  #Refine temperature
  var1 = ((((temp_raw>>3)-(dig_T1<<1)))*(dig_T2)) >> 11
  var2 = (((((temp_raw>>4) - (dig_T1)) * ((temp_raw>>4) - (dig_T1))) >> 12) * (dig_T3)) >> 14
  t_fine = var1+var2
  temperature = float(((t_fine * 5) + 128) >> 8);

  # Refine pressure and adjust for temperature
  var1 = t_fine / 2.0 - 64000.0
  var2 = var1 * var1 * dig_P6 / 32768.0
  var2 = var2 + var1 * dig_P5 * 2.0
  var2 = var2 / 4.0 + dig_P4 * 65536.0
  var1 = (dig_P3 * var1 * var1 / 524288.0 + dig_P2 * var1) / 524288.0
  var1 = (1.0 + var1 / 32768.0) * dig_P1
  if var1 == 0:
    pressure=0
  else:
    pressure = 1048576.0 - pres_raw
    pressure = ((pressure - var2 / 4096.0) * 6250.0) / var1
    var1 = dig_P9 * pressure * pressure / 2147483648.0
    var2 = pressure * dig_P8 / 32768.0
    pressure = pressure + (var1 + var2 + dig_P7) / 16.0

  # Refine humidity
  humidity = t_fine - 76800.0
  humidity = (hum_raw - (dig_H4 * 64.0 + dig_H5 / 16384.0 * humidity)) * (dig_H2 / 65536.0 * (1.0 + dig_H6 / 67108864.0 * humidity * (1.0 + dig_H3 / 67108864.0 * humidity)))
  humidity = humidity * (1.0 - dig_H1 * humidity / 524288.0)
  if humidity > 100:
    humidity = 100
  elif humidity < 0:
    humidity = 0

  return temperature/100.0,pressure/100.0,humidity

def main():

  # (chip_id, chip_version) = readBME280ID()
  # print("Chip ID     :", chip_id)
  # print("Version     :", chip_version)

  temperature,pressure,humidity = readBME280All()

  print(round(temperature,1),round(humidity,1),round(pressure,1))

if __name__=="__main__":
   main()

@JohnGalt said in AHT20 Humidity + Temperature Sensor:

Remote I/O error

The given error seems to by not a program but an I/O problem.
I’m not familiar with this sensor so I do not know how to connect and - most important - how to figure out the right “deviceAddress” - it seems that there is the error located.

Either the address isn’t the correct one or the IO channel is somehow broken.
For the latter came in my mind: if this is a I2C conected device - do you have enabled I2C on your Raspi?

I’ve tried to “understand” what happens in the python script above as well as in the C-exmaple program on the Bosch product page for bme280 sensor, but didn’t got it…

Regards,
Ralf

rkorell · Apr 16, 2025, 10:38 AM

@JohnGalt
just seen while re-reading my last posting:

nodehelper.js (the part of the module which is communicating with the python program and so with the sensor) is calling python3 :

exec(`python3 ./modules/MMM-BME280/bme280.py ${deviceAddr}`, (error, stdout) => {

but the python script itself is referencing python 2 (see above post, source code of bme280.py, 1st line:

#!/usr/bin/python

I guess this must match and doesn’t …
So may this is the cause of your problems.

And if this is true it is may really hard to get this working because you may run in several incompatibilities and library-conflicts …

Regards,
Ralf

JohnGalt · Apr 16, 2025, 9:47 PM

@sdetweil – Thanks, Sam. It looks like I intended to send bme280.py but actually copied MMM-BME280.js. I see below where Ralf did post it, so I will look at that, too.

JohnGalt · Apr 16, 2025, 9:57 PM

@rkorell – Hi Ralf:

Yes, I2C is enabled and functioning. There is a test script that does return current temperature and humidity:
Invoking “python AHT20_test.py” does launch this script (Note - I launch from the terminal using ‘python’, not ‘python3’, telling me the system is indeed defaulting to python3):

# SPDX-FileCopyrightText: 2021 ladyada for Adafruit Industries
# SPDX-License-Identifier: MIT

"""
Basic `AHTx0` example test
"""

import time
import board
import adafruit_ahtx0

# Create sensor object, communicating over the board's default I2C bus
i2c = board.I2C()  # uses board.SCL and board.SDA
# i2c = board.STEMMA_I2C()  # For using the built-in STEMMA QT connector on a microcontroller
sensor = adafruit_ahtx0.AHTx0(i2c)

while True:
    print("\nTemperature: %0.1f C" % sensor.temperature)
    print("Humidity: %0.1f %%" % sensor.relative_humidity)
    time.sleep(2)

JohnGalt · Apr 17, 2025, 1:01 AM

@rkorell + @sdetweil - Thanks for the support. I think I have fixed this for myself by adapting an existing module (“MMM-Temperature”, // https://github.com/Tom-Hirschberger/MMM-Temperature).

That module calls scripts for various sensors to capture the data. I was able to adapt an existing script for use with this sensor, so I think I am good for now.

rkorell · Apr 17, 2025, 6:25 AM

@JohnGalt cool.
congratulations.
It’s may be worth to show the community what you have done?

Regards,
Ralf

JohnGalt · Apr 17, 2025, 4:24 PM

@rkorell Yes, I can describe how I arrived at a resolution for my particular problem - which was to replace DHTxx sensors with the more accurate AHT20 temperature and humidity sensors.

Caveat: I do not necessarily recommend this for the average person though, because I did not find Magic Mirror modules that natively support this sensor, and as a result had to make changes outside of the config.js and custom.css files. As usual, this puts me at risk of something breaking when the module is updated.

The module being used (MMM-Temperature - found at https://github.com/Tom-Hirschberger/MMM-Temperature) utilizes python scripts to capture the data from the sensor. While the module is quite complete, with script support for many sensors, I did create a new script in order to use this particular sensor.

The sensor is supported by Adafruit (https://learn.adafruit.com/adafruit-aht20/python-circuitpython), with instructions to install various adafruit libraries including adafruit-ahtx0 (sudo pip3 install adafruit-circuitpython-ahtx0).

Using those libraries, your script needs to include the following instructions:

import board
import adafruit_ahtx0
sensor = adafruit_ahtx0.AHTx0(board.I2C())

One of the existing scripts supports I2C sensors using the Adafruit libraries, so it looked like a good candidate to use as a model. See htu21:

#!/usr/bin/env python3
#pip3 install adafruit-circuitpython-htu21d
import board
from adafruit_htu21d import HTU21D
import json

result = {}
try:
    # Create sensor object, communicating over the board's default I2C bus
    i2c = board.I2C()  # uses board.SCL and board.SDA
    sensor = HTU21D(i2c)
    result["temperature_c"] = sensor.temperature
    result["humidity"] = sensor.relative_humidity
    result["temperature_f"] = (result["temperature_c"]*1.8) + 32
    result["error"] = False
except:
    result["temperature_c"] = 0.0
    result["humidity"] = 0.0
    result["temperature_f"] = (result["temperature_c"]*1.8) + 32
    result["error"] = True

print(json.dumps(result))

My resulting script is aht20:

#!/usr/bin/env python3
# aht20: Modeled on htu21 
# pip3 install adafruit-circuitpython-htu21d
# pip3 install adafruit-circuitpython-ahtx0
import board
import adafruit_ahtx0
import json

result = {}
try:
    # Create sensor object, communicating over the board's default I2C bus
    i2c = board.I2C()  # uses board.SCL and board.SDA
    # sensor = HTU21D(i2c)
    sensor = adafruit_ahtx0.AHTx0(board.I2C())
    result["temperature_c"] = sensor.temperature
    result["humidity"] = sensor.relative_humidity
    result["temperature_f"] = (result["temperature_c"]*1.8) + 32
    result["error"] = False
except:
    result["temperature_c"] = 0.0
    result["humidity"] = 0.0
    result["temperature_f"] = (result["temperature_c"]*1.8) + 32
    result["error"] = True

print(json.dumps(result))

This module is now displaying the temperature and humidity from the sensor directly connected it it. Now all I have to do is figure out the complicated css and get it to look like the other modules on my Magic Mirror.

Barring any objections, I will mark this as solved.

sdetweil · Apr 17, 2025, 4:34 PM

@JohnGalt css, use the developers window

see the second link in my signature below

rkorell · Apr 17, 2025, 4:34 PM

@JohnGalt Really COOL!
Thanks for sharing!
Ralf

JohnGalt · Apr 17, 2025, 4:47 PM

@sdetweil – Thanks for the reminder, I’ll take a look.